1. Field of the Invention
The present invention relates to an implantable tubular prosthesis having a textile substrate with a fluid-tight microporous lining.
2. Description of the Prior Art
Tubular prostheses are commonly used as vascular grafts to replace damaged or diseased veins and arteries. To maximize the effectiveness of any prosthesis, it is desirable that the prosthesis have characteristics which closely resemble that of the natural body lumen which it is replacing.
Presently, conventional tubular prostheses and, more specifically, vascular grafts are formed by either weaving, knitting or braiding synthetic fibers into a tubular structure or using a polymer such as polytetra-fluoroethylene to create a tubular structure for use as a prosthesis. Tubular textile structures have the advantage of being naturally porous, which allows desired tissue ingrowth and assimilation into the body. Porosity must be balanced to allow for ingrowth of surrounding tissue, yet minimize leakage during the initial implantation. Attempts to control porosity and provide a sufficient fluid barrier have focused on tighter stitch construction such as knitted or woven double-velours and biodegradable natural coatings such as collagen or gelatin. While these grafts sought to overcome the difficulties in achieving the porosity/fluid-tight balance, they failed to adequately address the natural tendency of tubular structures to kink or collapse when the graft is twisted or bent during or subsequent to implantation. Thus, the prior art solutions to the porosity/fluid-tight balance left unanswered the problems of kinking and overall handling.
One conventional solution to the kinking and collapsing problems has focused on the reinforcement of the prosthesis walls using reinforcing fibers, rings, or bands circumferentially placed on the tubular structure. Additional reinforcement of this kind, however, has the disadvantage of reducing the radial and/or longitudinal compliance of the graft due to the increased stiffness of the reinforcing member. A reduction in compliance reduces the area through which blood can flow, thereby compromising the ability of the prosthesis to adjust to body conditions and perform naturally. Additionally, reinforcing members are generally made from solid structural materials which cannot be penetrated by cellular ingrowth from surrounding tissue and may even cause the erosion of the surrounding tissue during contraction.
Another method of increasing the kink and crush resistance of textile grafts is to crimp the graft, i.e., longitudinally compress the tubular structure. Crimping is generally described in U.S. Pat. No. 3,142,067. While crimping serves to add a dimension of kink and crush resistance to the graft, the intraluminal surface formed by crimping includes peaks and valleys which create hemodynamic turbulence within the graft as blood passes therethrough. This turbulence affects the rate of flow and the peaks and valleys formed on the intraluminal surface contribute to excessive thrombus formation and deposition of plaque.
Another disadvantage of presently available tubular textile prostheses, in particular woven and braided grafts, is that sutures tend to pull out or tear the fabric thereby making it difficult to attach the prosthesis to the existing body lumen and to prevent leakage at this junction. Furthermore, textile tubular prostheses formed from a synthetic yarn tend to have ends of the tube which easily ravel. Once the ends ravel or fray, suturing to the existing body lumen becomes extremely difficult.
Microporous tubing formed by stretching polytetra-fluoroethylene (PTFE) has also been used as implantable prostheses and especially as vascular grafts. PTFE porous tubes are considered by some to be superior in certain respects to conventional prostheses made of knitted or woven fabrics. The stretched or expanded PTFE tube has a microfibrous structure defined by the presence of nodes inter-connected by fibrils. While PTFE grafts have the advantage of being generally fluid-tight without the use of pre-clotting or specialized coatings, these grafts have limitations in their tear and tensile strength and compliance properties. PTFE grafts often require wrapping with a reinforcing support film to improve undesirable dilation. Reinforcement materials tend to impede the ingrowth of tissue necessary for rapid healing. In addition, PTFE grafts tend to be noncompliant as compared to textile grafts and natural vessels, thereby lacking many of the mechanical properties advantageous to textile grafts.
From the previous discussion it is apparent that both conventional textile prostheses and PTFE prostheses have respective benefits and disadvantages, but neither offers properties which solve all of the aforementioned problems.
Accordingly, it would be advantageous to provide a new and improved implantable tubular prosthesis which combines the best attributes and properties of each of the conventional grafts. More specifically, it would be particularly desirable to form a prosthesis which has the following characteristics: an outer surface porosity which encourages tissue ingrowth into the prosthesis; ravel and fray resistance for better suture retention and tailoring; longitudinal compliance for ease of implantation, sizing and natural vessel simulation; and a fluid-tight lumen without the need for pretreating, coating or pre-clotting.
The present invention addresses aforementioned the problems associated with the prior art and provides a soft-tissue implantable prosthesis in the form of a composite structure including a textile substrate and an integrated polymeric liner. The textile substrate includes an intraluminal surface having the liner affixed thereto, thereby rendering the tubular prosthesis fluid-tight. Accordingly, the outer surface formed by the textile substrate has the advantage of sufficient pore size to enhance tissue ingrowth and promote healing as well as other advantages associated with textile prostheses, such as flexibility and kink resistance. The liner provides a smooth, fluid-tight lumen to enhance fluid flow and which is made from a polymeric material which is naturally antithrombogenic.
The liner formed in accordance with the present invention is preferably formed from a polymeric material. Typical polymers for use in making the liner include, but are not limited to polytetrafluoroethylene, urethanes, silicones and polyesters. Preferably, expanded polytetrafluoroethylene is used to form the liner thus creating a microporous structure. The liner wall thickness need only be thick enough to provide a fluid-tight barrier to the intraluminal surface of the textile substrate. Thus, liner wall thicknesses are preferably thin and on the order of about 10 to about 50 microns.
The textile substrate formed in accordance with the present invention may be made by weaving, knitting or braiding yarns to form a tubular structure. In the preferred embodiment, the composite prosthesis including the textile substrate and the liner is heat conditioned to fuse the liner to the textile substrate. Thus, in one embodiment the textile substrate is formed from fibers having a melting temperature and bonding compatibility substantially similar to a material forming the liner.
In an alternative embodiment, the textile substrate may include a fusible fiber having a low melting temperature, the fusible fiber flowing onto the liner when melted for securing the liner to the textile substrate. In the case where expanded PTFE is used to form the liner, it is preferable that the fusible fiber have melt flow properties which allow the melted fiber to flow into the pores of the liner to secure the liner to the textile substrate.
In yet another embodiment, the textile substrate may be formed from any known fiber and the liner may be affixed to the substrate using an adhesive, by sewing the components together or by any other mechanical coupling means.
The textile substrate formed in accordance with the present invention may be formed by warp knitting to create a velour surface. The loops forming the velour surface are preferably on a the exterior surface to create a single-velour fabric. Single-velour fabrics have many favorable properties with respect to porosity, compliance, and suture retention.
Furthermore, the textile substrate may be formed from yarns, rovings, tapes or other stranded materials. Some of the yarns may be bioabsorbable while other yarns are merely biocompatible. Bioabsorbable yarns are preferably used to create an initial porosity different from the porosity once the bioabsorbable material has been absorbed into the body. For example, once a bioabsorbable yarn is absorbed into the body, a void or pore remains in its place. Additionally, the yarns used to form the textile substrate may be flat, twisted, textured or preshrunk.
The invention is also directed to a process for preparing a soft-tissue prosthesis. The process includes the steps of winding a polymer over a smooth mandrel to form a cylindrically-shaped liner, positioning a tubular textile substrate over an outer surface of the liner, heating the liner and textile substrate to a temperature sufficient to melt a portion of one of either the textile substrate or liner and cooling the textile substrate and liner thereby fusing the liner to the textile substrate.
In one embodiment, the textile substrate may be formed from a material which has a similar melting temperature and bonding compatibility to that of the liner. In an alternative embodiment, the liner is formed from expanded PTFE and the textile substrate includes a meltable yarn such that the molten yarn flows into the pores of the liner and, when cooled, fuses the liner to the textile substrate.
The present invention is also directed to a method of repairing a diseased blood vessel of a patient. The method includes removing a diseased portion of a blood vessel from the patient leaving a first and second open end of the blood vessel, inserting a tubular prosthesis between the first and second end of the blood vessel, the tubular prosthesis being formed from a textile substrate having an intraluminal surface and a liner affixed to the intraluminal surface of the textile substrate such that the liner renders the tubular prosthesis blood-tight, and securing the tubular prosthesis to the first and second open ends of the blood vessel to form a continuous lumen through which blood may flow.
Thus, the present invention overcomes many of the shortcomings associated with prior art soft-tissue prostheses. The soft-tissue prosthesis formed in accordance with the present invention utilizes the advantages of both a textile prosthesis and a polymer prosthesis to create a composite structure having a smooth, fluid-tight intraluminal surface yet providing a porous outer structure to encourage ingrowth of connective tissue and promote healing.
A preferred form of the textile substrate having a fluid-tight liner, as well as other embodiments, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.